The research doesn't confirm that exercise directly leads to improved rest, and it's possible there may be another explanation for the apparent connection between exercise and sleep. Still, the findings are mostly consistent with previous research, said Matthew P. Buman, an assistant professor of exercise and wellness at Arizona State University who's familiar with the study.

But if you think a daily walk or jog will clear up your sleep problems, that might be a bit too optimistic.

"In general, the relationship between physical activity and sleep is moderate," Buman said.

More than one-third of U.S. adults have trouble falling asleep at night or staying alert during the day, according to background information in the study. Inadequate sleep has been linked to depression, cardiovascular disease and other health problems.

The new study, led by researchers at Oregon State University, looked at statistics from a U.S. health survey conducted from 2005 to 2006. The researchers focused on more than 2,600 men and women -- aged 18 to 85 -- who measured their activity levels and answered questions about sleep.

All wore accelerometers, devices that measure physical activity, for a week.

The researchers adjusted their statistics so they wouldn't be thrown off by unusually high or low numbers of people of certain ages, weight, health condition, smoking history or other factors.

The researchers then determined how many participants met or exceeded national exercise guidelines by getting at least 150 minutes a week of moderate exercise or 75 minutes a week of vigorous exercise or a combination of both.

Those who met the guidelines were 65 percent less likely to report often feeling sleepy during the day compared to those who got less exercise. They were also 68 percent less likely to report sometimes having leg cramps and 45 percent less likely to report having trouble concentrating while tired.

The study appears in the December issue of the journal Mental Health and Physical Activity.

Buman called the finding intriguing, even if it doesn't prove that exercise improves sleep.

If that is the case, however, the causes are unclear. Some researchers think physical activity improves sleep by helping reduce levels of stress, anxiety and depression, he said. "Others have suggested an energy conservation hypothesis, essentially saying that when you burn more calories through exercise, your body more efficiently uses the sleep period to recover. Others have suggested that exercise can modestly reduce body weight, which in turn helps people to sleep better."

Another theory suggests that exercise helps the body deal better with the cooling down of its temperature during sleep, he said.

So should you avoid exercising before bed, as conventional wisdom suggests? The new research doesn't look at the timing of exercise, but the study authors do note that most previous studies haven't shown that late-night exercise disrupts sleep quality.

Study co-author Brad Cardinal, a professor of exercise science at Oregon State, said in a statement that the study is unusual because it directly measured how much exercise people got instead of relying on their memories, which can be faulty.